Yeah I know how you feel and I don't know if I should try to give advice on this, since I have a hard time myself staying calm in confrontational situations, but here's the few tricks I found so far to keep a certain control:
Before the meeting, try to put your ideas in order so that they come out more easily.
Make clear, short sentences and write them down. Try to memorize them.
Take slow, deep breath to calm yourself down before going to your meeting. Try to put yourself in the mindset that today, you're the most easy going person in the world.
Keep breathing slow and deep as you go to your meeting, and bring your piece of paper.
Keep breathing slow and deep during the meeting.
Don't look the person in the eyes when you speak if it makes you lose your calm and forget your ideas. You can look at them when you're done speaking or when you say the last words of your sentence.
Don't waste your energy trying to cut people while they are speaking, unless really necessary. Just wait for your turn, and keep breathing slow and deep while you listen.
When they finally shut up, say your clear, short sentences. Try to speak clearly and not too fast. Look at your paper if needed. It might look odd, but who cares.
For anything that is not on your paper, take your time to think and breathe before answering, even if the person in front of you look impatient.
That's all I got for now. It's been working well enough for me.
